Screw fastener joining of machine elements in practice results in subjecting the machine elements to a clamping force effectuated by one or more individual screw fasteners. The magnitude of the clamping force is dependent on screw fastener dimensions and to what extent the individual screw fastener is tightened, whereby typically reaching of a predetermined tightening torque shuts off the screw tightening tool.
A particular problem is encountered in connection with using screw fasteners for fastening of rigid machine elements such as e.g. rigid steel plates, to each other. The reason for this is that in such cases it might be the case that at the moment of screw driver shut down, because of reached target toque, the fastener is not sufficiently tightened for the machine elements to be fastened tightly against each other. Instead, in the region of the tightened screw fastener, there exists a certain play between the rigid plates because of lack of contact between them in that position. Screw fasteners tightened in such a way are problematic, since they are likely to prematurely break from fatigue due to the fact that a fluctuating load will be taken up by the fastener rather than reducing the clamp load between the parts.
In order to remedy this problem, it has been suggested to position shim elements between the metal plates in order to bridge the play. This method is, however inexact, unreliable and also labour intensive.
Another problem with previously known screw tightening systems is that friction occurring between surfaces of the screw fastener varies considerably, whereby the same shut off torque can lead to great variation of clamping force between different screw fasteners. Since the clamping force typically is realised to vary by 35%, a screw fastener is typically tightened to only part of its capacity, as an example not more than 60% of the theoretical capacity in production screw tightening. This means that the screw fastener has to be over-dimensioned to reach a predetermined clamping force.
As a remedy to this problem it has been suggested to tighten screw fasteners to a point beyond the yielding point, (i.e. where plastic deformation occurs). Hereby the advantage is achieved that 100% of the screw fastener capacity can be used. The clamping force also then typically only varies with the hardening of the material in the screw fastener, which gives smaller variations.
The problem is now to verify that the point beyond the yielding point has been reached. For that purpose different methods have been developed, wherein, the tightening tool includes a measuring gauge in order to secure tightening quality. Another solution is to measure permanent elongation of the screw fastener after loosening it which, however, can give erroneous results since the results depend on the accuracy of a test screw. This method is also labour intensive. Another method is ultra sound measuring.